The preparation of cellulose ethers having uniform or different types of ether substituents is known [see, for example, "Ullmanns Encyklopaedie der technischem Chemie" (Ullmann's Encyclopedia of Industrial Chemistry), Verlag Chemie--Weinheim, 4th edition, 1975, pages 192 et seq.]. Each preparation is, in general, either by x) the principle of Williamson's ether syntheses [by reacting cellulose with alkyl halides or aralkyl halides (with stoichiometric consumption of a base)] and/or by y) reacting cellulose with activated reactants which are capable of reacting directly with hydroxyl groups (in the presence of catalytic, i.e. sub-stoichiometric, quantities of a base): ##STR1##
In these general equations:
Cell-O-H denotes a hydroxyl group (on the cellulose molecule) which is to be etherified, PA1 Hal denotes chlorine or bromine, PA1 R.sup.1 denotes an alkyl radical (from C.sub.1 to C.sub.15), an aralkyl radical (from C.sub.7 to C.sub.15), a carbonylalkyl radical (from C.sub.1 to C.sub.3), a sulfonoalkyl radical (from C.sub.1 to C.sub.3), a phosphonoalkyl radical (from (C.sub.1 to C.sub.3), a hydroxyalkyl radical (from C.sub.1 to C.sub.6) or an N,N-dialkylaminoalkyl radical (in which each alkyl group is from C.sub.1 to C.sub.3), PA1 BOH denotes a base, such as NaOH, an alkali-metal hydroxide, an alkaline-earth-metal hydroxide or a quaternary ammonium base.
each of R.sup.2 and R.sup.3 is, independently, hydrogen or an alkyl radical (from C.sub.1 to C.sub.13), and
For preparing mixed ethers of cellulose, several etherifying agents are allowed to act simultaneously or stepwise on cellulose. For this purpose, reactions are carried out according to only one of the variants [(x) or (y)] indicated or, preferably, according to both variants. The following are examples of reaction products which are prepared by variant (x): methyl cellulose (MC), benzyl cellulose (BC), carboxymethyl cellulose (CMC), sulfonoethyl cellulose (SEC), phosphonomethyl cellulose (PMC), or N,N-diethylaminoethyl cellulose (DEAEC). The following are examples of reaction products which are prepared by variant (y): hydroxyethyl cellulose (HEC) or hydroxypropyl cellulose (HPC). Mixed ethers of cellulose which are prepared by either one or both of the variant(s) include, for example, methyl hydroxethyl cellulose (MHEC), ethyl hydroxyethyl cellulose (EHEC), hydroxyethyl hydroxypropyl cellulose (HEHPC), methyl carboxymethyl cellulose (MCMC), hydroxyethyl phosphonomethyl cellulose (HEPMC), or methyl hydroxyethyl hydroxypropyl cellulose (MHEHPC). Within the scope of the following statements, the term "cellulose ethers" is understood to mean both products having a unitary substituent, such as hydroxyethyl cellulose, and products having at least two different substituents, such as methyl carboxymethyl cellulose.
The former, not prepublished, German Offenlegungsschrift No. 33 16 124 (filed May 3, 1983) describes a process for preparing water-soluble mixed cellulose ethers in three steps, comprising:
(a) alkalization of cellulose,
(b) etherification of the alkali cellulose in the presence of a base, using at least one etherifying agent which requires, for the reaction with cellulose, a catalytic and sub-stoichiometric quantity of a base, and (after increasing the quantity of base)
(c) further etherification of the cellulose ether so prepared, using at least one etherifying agent which requires, for the reaction with cellulose, an at least stoichiometric quantity of a base or a catalytic and substoichiometric quantity [in excess of the quantity used in (a)] of a base,
with at least one inert solvent selected from the group of dimethoxyethane, an alkanol, an alkane diol and an alkoxy alkanol; the inert solvent being used as a dispersing auxiliary in at least one of the steps and water being present in all steps. Apart from dimethoxyethane, these dispersing auxiliaries include, in particular, alkanols [from C.sub.1 to C.sub.5 (p.e., isopropanol)], alkane diols [from C.sub.2 to C.sub.3 (p.e., ethylene glycol)], and alkoxy(C.sub.1 to C.sub.4)alkan(C.sub.2 or C.sub.3)ols (p.e., methoxyethanol). The etherifying agents used are, for example, ethylene oxide in step (b) and methyl chloride or monochloroacetic acid in step (c). For further details and the discussion of the respective state of the art, reference is made to the cited German Offenlegungsschrift.
Processes which differ considerably from the preceding employ dimethyl ether as an organic solvent. Processes of this kind are, for example, disclosed by:
British Patent No. 909,039, which describes a process for the production of MC, MHEC or MHPC, in which dimethyl ether or diethyl ether is employed; the alkali content of the reaction mixture is not changed; no synthesis of a cellulose mixed ether is described in the Examples; the quantity of dimethyl ether used is about 2 to 3 times more than the weight of the cellulose, and the organic solvent is not present during the alkalization step;
German Patent No. 15 43 136 (=U.S. Pat. No. 3,544,556), which describes a process (for the production of MC, MHEC or MHPC) in which dimethyl ether (formed as a byproduct of the etherification) is used; the alkali content of the reaction mixture is not changed; a quantity of about 0.3 to 0.4 part by weight of dimethyl ether is used in the Examples per 1 part by weight of cellulose; and organic solvent is not present in the alkalization step;
German Offenlegungsschrift No. 26 36 935 (=U.S. Pat. No. 4,015,067), which describes the production of various cellulose ethers, and in which excess alkyl chloride, dimethyl ether, hexane, benzene or toluene are said to be used as organic solvents; the alkali content of the reaction mixture is not changed; the only organic solvent used in the Examples is excess alkyl chloride; all reactants are first mixed in a vessel, and the total weight of the etherifying agent plus the organic solvent is about 10 to 20 times higher than the weight of the cellulose, whereby the dimethyl ether portion should not exceed 60% of the total.